1. Field of the Invention
The present invention relates a sheet fed apparatus for an image forming system and, more particularly, to a sheet feed apparatus for feeding each sheet (i.e., a cut sheet such as a copying sheet, a transfer sheet, or a recording sheet) stored in a sheet storage unit to a main unit (e.g., a copying machine or printer) for receiving each sheet and performing predetermined processing such as image formation on the received sheet.
2. Related Background Art
A typical example of the main unit and sheet feed apparatus described above is shown in FIG. 1. FIG. 1 shows a laser beam printer (LBP) from which a process cartridge and a sheet cassette are detachable.
The laser beam printer includes a printer housing 100, and an image forming mechanism is incorporated in the printer housing 100. The housing 100 is divided into upper and lower halves 100B and 100A. The upper half 100B can be pivoted about a rear end hinge shaft 101 upward with respect to the lower half 100A and can be open, as indicated by the alternate long and short dashed line. The upper half 100B can be pivoted downward and can be closed with respect to the lower half 100A, as indicated by the solid line in FIG. 1.
The upper half 100B is always biased by a spring means (not shown) toward an open direction. When the upper half 100B is completely closed with respect to the lower half 100A against the biasing force of the spring means, a locking mechanism (not shown) is operated to lock the upper half 100B with the lower half 100A. As a result, a closed state can be appropriately maintained, as indicated by the solid line.
When the locking mechanism is released, the upper half 100B is opened by the biasing force of the spring means, as indicated by the alternate long and short dashed line and is held at a predetermined open angle. Most of the interior of the printer can be observed by an operator.
The upper half 100B is raised as described above to largely expose the interior of the printer so as to mount a process cartridge 102 in the printer and perform inspection and maintenance of the interior of the printer.
An open side (left end face in FIG. 1) of the upper half 100B of the printer is its front side. The hinge shaft 101 is located at the rear side (i.e., the right end face in FIG. 1) of the printer.
The process cartridge 102 has a cartridge housing 102a which contains four process units, i.e., a photosensitive drum 103, a charger 104, a developing unit 105, and a cleaner 106. The process cartridge 102 is detachable from a predetermined storage section at about the central portion of the upper half 100B. The cartridge 102 is mechanically and electrically connected to the printer such that mechanical drive and electrical circuit systems of the cartridge and the printer are connected through a coupling member (not shown). A laser scanner unit 107 and a laser reflecting mirror 108 are arranged at predetermined positions inside the upper half 100B.
A sheet cassette 109 is defined as a sheet storage unit. The cassette leading end of the sheet cassette 109 is inserted from a cassette insertion opening 100C formed at the lower front end face of the lower half 100A of the printer. When the sheet cassette 109 is sufficiently inserted in the X direction inside the lower half 100A along a base plate 100D in the lowere half 100A, a boss 109f formed on the outer bottom surface of the cassette is engaged with an engaging recess 100E formed on the upper surface of the base plate 100D, so that the cassette 109 is positioned and mounted in the printer. When the sheet cassette 109 is pulled against the engaging force of the boss 109f and the recess 100E in the Y direction, the cassette 109 can be pulled outside the printer.
The sheet cassette 109 includes a main case 109a and an intermediate plate 109b located inside the case 109a. The rear side of the intermediate plate 109b is biased by a spring 109d to pivot upward about a pivot portion 109c. Sheets (i.e., transfer sheets) S are stacked on and stored in the main case 109a. The sheet S extend on the upper surface of the intermediate plate 109b and the inner bottom surface of the main case 109a. The level of the upper surface of the stack of sheets S is kept unchanged regardless of a decrease in sheets upon feeding of each sheet S or the number of stacked sheets since the intermediate plate 109b is basied upward by the spring 109d.
A separation pawl 109e is engaged with the leading ends of the upper surfaces of the sheets S to separate one sheet from the stacked sheets.
A sheet feed system includes a sheet pickup roller 110, a pair of convey rollers 111a and 111b for conveying the sheet picked up by the sheet pickup roller 110, sheet guide plates 112, 113, and 115, a pair of registration rollers 114, a transfer charger 116, a sheet conveyor belt 117, and a pair of sheet delivery rollers 110. The printer also includes a fixing unit 118. The members 110 to 119 are located at predetermined positions in the lower half 100A.
A sheet delivery opening 100D is formed in the upper front surface of the lower half 100A. The pair of sheet delivery rollers 119 oppose the opening 100D. A delivery sheet tray 100F is mounted in the printer.
When the upper half 100B in which the process cartridge 102 is mounted is closed with respect to the lower half 100A, as indicated by the solid line, the lower surface of the photosensitive drum 103 on the process cartridge 102 side opposes at a normal position the transfer charger 116 mounted in the lower half 100A and is spaced apart therefrom by a predetermined distance. When the sheet cassette 109 is normally mounted in the lower half 100A, the upper surface of the leading end of the stack of sheets S opposes the lower position of the sheet pickup roller 110.
When the image formation start signal is input to a printer control system, the photosensitive drum 103 is rotated at a predetermined peripheral velocity in a direction indicated by an arrow. The outer surface of the photosensitive drum 103 is uniformly charged by the charger 104 to a predetermined polarity (i.e., a positive or negative polarity).
The laser scanner unit 107 generates a pixel laser beam L corresponding to a timer-serial electrical pixel signal of image information input from an original image photoelectric reading unit (not shown), a word-processor (not shown), or a computer (not shown).
The laser scanner unit 107 then scan a surface of the photoelectric drum 103. That is, the uniformly charged surface of the rotating photosensitive drum 103 is sequentially scanned with the laser beam L from the laser scanner unit 107 through the mirror 108 and an exposure window 102b formed in the cartridge housing 102a in the axial direction of the drum. Therefore, a latent image corresponding to image information is formed on the photosensitive drum 103.
The formed latent image is visualized as a tone image by the developing unit 105, and the toner image is transferred to the sheet S fed to a position of the transfer charger 116 between the photosensitive drum 103 and the charger 116.
A convey force is applied to the uppermost sheet S stored in the sheet cassette 109 upon counterclockwise rotation of the pickup roller 110. The leading side of the sheet is moved from a position below the separation pawl 109e to a position above the separation pawl 109e. Only the upermost sheet is picked up and fed in the same direction as the mounting direction of the cassette 109. The sheet is guided by the guide plate 112 and is relayed and conveyed by the pair of convey rollers 111a and 111b. The sheet is then U-turned toward the registration rollers 114 located above the sheet pickup roller 110 in a direction (i.e., from right to left in the printer) opposite to the direction for picking up the sheet from the cassette 109. The U-turned sheet is fed by the registration rollers 114 between the photosensitive drum 103 and the transfer charger 116 through the guide plate 115 from the right in synchronism with rotation of the photosensitive drum 103. Therefore, the toner image formed on the photosensitive drum 103 is transferred to this sheet.
The sheet which received the toner image during feeding of the sheet between th photosensitive drum 103 and the charger 116 from the right to the left is separated from the surface of the photosensitive drum 103 by a separating means (not shown). the sheet is then conveyed in the fixing unit 118 by the conveyor belt 117, and the toner image is fixed. The sheet is then delivery as an image-formed product (print) on the tray 100F through the pair of delivery rollers 119 and the delivery opening 100D. After toner image transfer, a residual toner on the surface of the photosensitive drum 103 is removed by the cleaner 106, and the drum surface is cleaned to prepare for the next image formation cycle.
A rear surface plate 120 of the lower half 100A of the printer can be pivoted downward about a hinge portion 121, as indicated by the alternate long and two short dashed line in FIG. 1. The curved guide plate 112 and the roller 111b of the pair od sheet convey rollers 111a and 111b are disposed inside the rear surface plate 120. When the sheet is jammed in a U-turned sheet path from the sheet pickup roller 110 to the registration roller 114, the rear surface plate 120 is moved down and opened to remove the jammed sheet.
The conventional laser printer described above has the following problems.
(1) Since the feed direction of the sheet S picked up by the sheet pickup roller 110 from the sheet cassette 109 serving as a sheet storage unit inserted in the printer as the main unit is the same as a direction X for inserting the sheet cassette 109 into the printer, the position of the sheet with respect to the cassette is the cassette insertion/mounting leading end. The size of the cassette 109 varies depeneding on different lengths of sheet S. An extended portion of each cassette from the cassette insertion/removal opening 100C toward the front surface of the printer varies depending on different cassette sizes, resulting in poor appearance.
When a cassette 109' (indicated by the alternate long and two short dashed line) which stores longer sheets in the feed direction is mounted in the printer, an extended length P' of the rear end portion of the cassette is increased, and the space for installing the printer is undesirably increased.
(2) Since the sheet S in the cassette 109 is fed from the cassette insertion/mounting direction leading end in a cassette insertion/mounting direction, the fed sheet is U-turned in a direction opposite to the sheet pickup direction and is conveyed to the transfer unit as a sheet processing unit through the sheet guide plates 112 and 113. The frequency of occurrence of jamming in the U-turn guide plates 112 and 113 is higher than that in other portions in practise. In addition, since the U-turn guide plates 112 and 113 are located in the rear portion of the printer, an operator must walk to the rear side of the printer to open the rear surface plate 120 so as to eliminate jamming upon occurence of jamming in the U-turn guide plates 112 and 113, resulting in inconvenience.
(3) The upper sheets S stored in the cassette may be separated above the separation pawl 109e by a shock upon mounting of the cassette 109 in the printer. These sheets are undesirably moved from the cassette front end in the sheet feed direction which is the same as the cassette insertion/mounting direction. This causes a feed error of the sheet and double sheet feeding.
(4) The sheet pickup roller 110, the pair of convey rollers 111a and 111b, and the guide plates 112 and 113 which constitute a sheet feeder unit feeding the sheet S from the sheet cassette 109 as the sheet storage unit to a predetermined unit (transfer unit) of the printer as the main unit are permanetly mounted in the lower half of the printer. If a sheet cassette except for a standard cassette and a paper deck are required due to sheets except for standard sheets, the lower unit including the above members must be replaced with a unit corresponding to the paper size.